WASHINGTON - Facing the darkest days of his presidency, President Bush is frustrated, sometimes angry and even bitter, his associates say.

With a seemingly uncontrollable insurgency in Iraq, the White House is bracing for the political fallout from a grim milestone that could come any day: the combat death of the 2,000th American G.I.

Last week alone, 23 military personnel were killed in Iraq, and five were wounded yesterday in a relentless series of attacks across the country.

This week could also bring a special prosecutor's decision that could shake the foundations of the Bush government.

The President's top political guru, Karl Rove, and Vice President Cheney's right-hand man, Lewis (Scooter) Libby, are at the center of a two-year criminal probe into the leak of a CIA agent's identity. Many Bush staffers believe indictments are likely.

"He's like the lion in winter," observed a political friend of Bush. "He's frustrated.

He remains quite confident in the decisions he has made. But this is a guy who wanted to do big things in a second term. Given his nature, there's no way he'd be happy about the way things have gone."

Bush usually reserves his celebrated temper for senior aides because he knows they can take it. Lately, however, some junior staffers have also faced the boss' wrath.

"This is not some manager at McDonald's chewing out the help," said a source with close ties to the White House when told about these outbursts. "This is the President of the United States, and it's not a pleasant sight.

Bush flashes the bird, something aides say he does often and has been doing since his days as governor of Texas.

The specter of losing Rove, his only truly irreplaceable assistant, lies at the heart of Bush's distress. But a string of political reversals, including growing opposition to the Iraq war, Hurricane Katrina's aftermath and Harriet Miers' bungled Supreme Court nomination, have also exacted a personal toll.

Presidential advisers and friends say Bush is a mass of contradictions: cheerful and serene, peevish and melancholy, occasionally lapsing into what he once derided as the "blame game." They describe him as beset but unbowed, convinced that history will vindicate the major decisions of his presidency even if they damage him and his party in the 2006 and 2008 elections.

At the same time, these sources say Bush, who has a long history of keeping staffers in their place, has lashed out at aides as his political woes have mounted.

"The President is just unhappy in general and casting blame all about," said one Bush insider. "Andy [Card, the chief of staff] gets his share. Karl gets his share. Even Cheney gets his share. And the press gets a big share."

The vice president remains Bush's most trusted political confidant. Even so, the Daily News has learned Bush has told associates Cheney was overly involved in intelligence issues in the runup to the Iraq war that have been seized on by Bush critics.

Bush is so dismayed that "the only person escaping blame is the President himself," said a sympathetic official, who delicately termed such self-exoneration "illogical."

A second senior Bush loyalist disagreed, saying Bush knows "some of these things are self-inflicted," like the Miers nomination, where Bush jettisoned contrary advice from his advisers and appointed his longtime personal lawyer.

"He must know that the way he did that, relying on his own judgment and instinct, was not good," another key adviser said.

Despite the turmoil, Bush is determined to soldier on, already preparing for two major overseas trips in November and helping shape next year's legislative agenda.

"I've got a job to do," he told reporters last week. "The American people expect me to do my job, and I'm going to."